I assume these are a little more sophisticated than a sheet of paper with the right person's signature. I'm picturing public-key cryptography: Johnny Traveller is going from Deneb to Procyon. The Bank of Deneb gives Johnny a file, or maybe even a piece of hardware containing a file, encrypted with the Bank of Procyon's public key. When he gets to Procyon, the Bank decrypts the file (maybe the device serves as proof of origin?), and deposits 5 MCred into Johnny's account.

The proof-of-origin problem is bugging me, though. Maybe the device is engraved at the molecular or atomic level, requiring extraordinarily expensive (and high-TL) tools to falsify?

I've just been using Imperium currency. And exchanging it for local currency when needed. My players aren't doing any mad dashes across sectors in our games, for bank accounts to have to keep up with their characters. Ships have safes with cash in them. And characters have mag-tech money on them. Some characters will just have their own life savings on a chip they have with them. No bank.

If players want to steal monies, they have the "crack into safe" or the "hack into chip" way. Depends on what their character would do.

The -1776 date for the end of the Rule of Man is arbitrary, and notes the financial
collapse of the central government, when the Treasury at Hub/Ershur refused
to honor a monetary issue of the branch treasury at Antares. The resulting lack
of confidence within monetary circles marked the end of large-scale interstellar trade
and of effective governmental power within the Rule of Man.

I have a much more interesting version IMTU that links in with the AI robots the Terrans were using at the end of the Interstellar War period

The Virus era was actually a horribly mutated bitcoin algorithm that went out of control. Spread across known space as people greedily hoped to see their monies' value explode then collapsed bringing down an empire. After that, they made up a ludicrous story about some 'living computer chip'.

The deyo chip was bred/engineered call it what you like from a living, sentient silicon based life form.

Also within Traveller now we have the ability to overwrite a personality with the digitised version of another personality.

So if you can take the digital personality of a sentient silicon based lifeform and weaponise it are you surprised that Virus could have such an effect? Computer systems that are based on principles we can't even guess at...

Would Virus have even been a threat if the Imperium wasn't already nearly at the apocalypse phase of Hard Times?

A living silicon based life form is more ludicrous than psionics, gravitics and the total lack of thermodynamics?

Citizens of high-technology (TL8 or higher) planets often use electronic Credits. Transactions are authenticated and managed using computers or personal comms. While electronic Credits work perfectly well in-system, lack of faster than light communications in some universes means it is possible for a Traveller to outrun his Credits – having a million Credits in one’s bank account on Sylea is all well and good, but if you are twenty parsecs away and the data has not caught up with you, then it is inaccessible. Large banks, corporations and other financial institutions use X-Boat networks or similar facilities to synchronise important financial records, but private citizens must make arrangements to keep Credits available. It is possible to notify one’s bank and have a line of Credit sent ahead (or at least sent at the same time) if you know your destination, but wandering Travellers must sometimes default to using physical cash or trade goods. In addition to physical Credits, precious metals, gemstones, radioactive elements or technological wonders are sometimes used as barter on many worlds.

Also on the same page...

It is assumed that the physical credit is minted by the dominant empire of the universe and is virtually impossible to falsify.

How common is it for worlds within the Imperium to have their own currency, separate from the Imperial credit?

Yes. The X-boat network would constantly be sending out updates on the banking system throughout the empire. It would be possible to out-run your cash stored on one planet, but there are a number of ways to accommodate that. Much like merchants did in the ye olde days, credit would be based on reputation and just how much cash/credit you were looking to draw out. Your bank on one world could issued a letter of credit that you could deposit elsewhere - it would work much like a bearer bond does today, though probably with a bit more security built in so it's not like taking cash. Cryptography could easily satisfy the need for security. A digital hash of say 12,000 characters might take the receiving bank a day or two to decode, but when you are talking about 10s of millions of credits, everyone involved knows that security trumps everything. For a letter of credit in that high of a denomination a receiving bank would have no real issue offering you a credit advance of say Cr25,000 to cover a day or two of expenses. Banks do that today based on the amount of a check being deposited and your banking history with them.

Future money would probably take the form of electronic/plastic currency like it is today, along with paper notes and coins. Japan has a huge electronic currency footprint with the ability to purchase things with your cellphone or credit card. But they still use physical currency as well. People like holding on to physical money, so it's probably likely it will continue in the future.

The game doesn't cover the concept of currency very well. In it a Cr is the fiat currency of the Imperium and it's worth the same from planet to planet. But we know that's not realistic when you have individual polities. The current world currency is a good example. Anywhere in the US a dollar retains it's value, though the actual cost of a good varies. In the Imperium it's likely that balkanized worlds, or those that are not close to the Imperium would have their own currencies. But that currency would have no real value beyond the world, or say the immediate jump range of that planet. To have value it needs to be readily exchangeable, and having to travel for weeks or months to exchange it would not be economical unless someone was offering a huge incentive to swap currency. Plus the Imperium is going to want to squash the ideas of easily converting currency beyond a world since the Imperium is tied together using the Imperial Credit. By not providing an easy way to convert planetary cash it make the Imperial Credit the defacto currency.

The Imperium requires the adoption of the Imperial Credit as currency.

Much the same way as the EU requires new member states adopt the Euro, and the US would require its colonies, territories and dependencies to be using the US dollar before they could apply for full membership as a state,

The Imperium requires the adoption of the Imperial Credit as currency.

Much the same way as the EU requires new member states adopt the Euro, and the US would require its colonies, territories and dependencies to be using the US dollar before they could apply for full membership as a state,

The currency issue (US vs. Euro) is an interesting argument, but one that really doesn't have a comparison on the US side. In the history of the US there has only been a single example of a nation joining the US (the Republic of Texas existed for a decade before joining). Other than that all US states that were once territories had US currency by default. Europe, however, has nations with their own currencies joining the European Union, which I guess is more like a confederacy. The four nations that joined the initial EU had, as part of the Maastrich treaty, the right to have their own currency. I am not sure if they can maintain an outside status indefinitely, or if there is some eventual time frame where they mush convert their currency over. The UK is of an economic size where that has actual meaning to the overall EU, but Denmark is pretty small and would be more of a speedbump than anything else.

For interplanetary trade the standard is the Imperial credit. However, the Imperium is more of a confederacy in that it allows planets very wide latitudes and even outright hostility towards offworlders - but there are lines that a planet can't cross.

There's nothing to say that a planet cannot have it's own internal currency and people trade it for Imperial credits at the starport demarcation line. That concept fits within the narrative of the Imperium. And since the Imperial credit is the fiat currency of the Imperium, trading it on other worlds isn't quite the same as you do with other currencies trading in say the dollar. If you don't have an official trade for it you can only make trades unofficially - which means the conversion rates are going to really suck and nobody will do it commercially. If you are already off-planet then you could have traded it officially at the starport, so why keep gobs of nearly useless currency on you?

Here are two earlier discussions of this topic. The simple answer is that on worlds with sufficient tech level, electronic transactions are dominant for in-system business (and high law worlds may even ban cash). For inter-system transactions cash is dominant except for transactions so large that it's worth hijacking a starship to rob the safe (or when the large denomination currency is too scarce); when cash is not used, high technology letters of credit are used instead.

For individuals traveling between systems on normal schedules, correspondent bank branches or cash are typical.

I don't see the need for large cash transfers for business between systems. That's what a little thing called credit is for. If merchants made it work in the age of sail then surely the 52nd century can do it. So I see no issue with letters of credit being transmitted via the normal banking system for businesses. A prime example of businesses trying to use large amounts of cash or gold to make a transaction work is Iran and it's oil business. As the international sanctions bit down and they were unable to access the normal banking system they tried to revert to using cash and gold. While some of the transactions worked, they were severely limited in their trading partners who would accept such things. More importantly, they took a hit on their profits because they could NOT access the normal electronic banking system.

Now, where you WILL see ships stuffed with specie (e.g. physical credits) is when there are transfers of cash from system to system, or even planet to planet. There is always going to be such a need since trade isn't static and people spend and take money on their own schedules. Armored cars.... in space!

Warren Buffett agrees with me: always have a large pool of cash at hand.

He also said only invest in what you understand (which is why he has almost no tech in his portfolio) and buy and hold. Though excepting his General Re and the other insurance companies, he stays away from banking.

I assume these are a little more sophisticated than a sheet of paper with the right person's signature. I'm picturing public-key cryptography: Johnny Traveller is going from Deneb to Procyon. The Bank of Deneb gives Johnny a file, or maybe even a piece of hardware containing a file, encrypted with the Bank of Procyon's public key. When he gets to Procyon, the Bank decrypts the file (maybe the device serves as proof of origin?), and deposits 5 MCred into Johnny's account.

The proof-of-origin problem is bugging me, though. Maybe the device is engraved at the molecular or atomic level, requiring extraordinarily expensive (and high-TL) tools to falsify?

This is pretty much how I envision it working. Governments (or agencies of governments, depending on how finely you want to slice it) and banks and other entities have public keys, which are published for anyone to access. Anyone who expects to deal with these entities will keep these public keys handy, or can access them from whatever secure and vetted cache they wish. These can be used to authenticate documents (including letters of credit) issued by said entities. While there will be some transfers of cash or valuable trade goods (gold, platinum, tulip bulbs--whatever), most interstellar trade will probably involve letters of credit and debits and credits on ledgers.

It is not a perfect system. But it's good enough that it can be relied upon for widespread trade.

The amounts of data that is indicated to be moved around are massive, requiring large specialized computers and sophisticated high speed communications arrays to be able to move the data from ship to planet, and back again. The equipment to be able to carry mail is 20 tons at TL13, although I don't understand the rational that it is larger at TL13 than it is at TL10.

Simple hand computers and data wafers are indicated to have ridiculous amounts of storage capability by today's standards. Who is to say that you couldn't transmit all of the banking transactions from system to system, updating at each stop. On the mains, I would imagine at least a daily X-Boat coming/going. more for sub-sector and sector capitals, and other trade centers. Off the beaten path, you might want to bring specie, trade goods, or a letter of credit.